Recently, a method based on a silicon (Si) processing technique for manufacturing an element for optical communication (i.e., a silicon photonics technique) is attracting attention; and technological development relating thereto has been actively carried out. In the silicon photonics technique, an optical integrated circuit is realized by combining minute optical elements and integrating them on a single substrate. More specifically, an optical integrated circuit is formed by integrating optical elements such as a multimode optical waveguide element, an optical switch, an optical modulator and so on by using optical waveguides formed on the substrate.
Some optical integrated circuits are made based on silicon-on-insulator (SOI) substrates, quartz-based PLCs (Planar Lightwave Circuit), and so on. A SOI substrate is a laminated substrate that is made by forming a silicon dioxide film, that is referred to as SiO2 buried oxide film (a so-called BOX layer), on a substrate such as a silicon substrate, and forming thereon a silicon film that is referred to as a silicon active layer. By processing the silicon active layer to shape it to be a narrow line, and forming a top clad layer having an index of refraction lower than that of Si, an optical waveguide, that comprises the silicon as a core and the BOX layer and the top clad layer as clad parts, can be formed. Note that, in the case that a quartz-based PLC is to be used, the core made of the silicon is also buried by the silicon dioxide.
Many of the optical elements of the optical integrated circuit are waveguide-type elements that can be easily miniaturized. Also, the most basic optical element is an optical waveguide; and the optical waveguides comprise a linear waveguide, a curved waveguide, a branching waveguide, and so on.
In the above silicon photonics technique, in general, a size of a cross section of signal light passing through an optical waveguide is less than 1 μm square. On the other hand, for example, in the case that a standard optical fiber for communication is set to be an external optical circuit, a diameter of signal light is equal to or more than 10 μm square; thus, a difference of 10 to 100 times in the beam diameters may be anticipated. That is, in the silicon photonics technique, a construction of a coupling part, such as how coupling between a silicon optical waveguide and an external optical circuit is realized, becomes a very important matter.
There is a technique to realize the above, wherein signal light is inputted from an upper plane or a lower plane of a plane of an optical integrated circuit by use of a grating coupler (refer to Non-patent-related Document 1). Also, in addition to the above, there is another technique to realize the above, wherein a coupling part is formed by use of an optical element such as a spot size converter (SSC), and the light is made to be entered substantially in parallel with a plane of the optical integrated circuit by carrying out end-surface coupling to optically couple an external optical circuit to a chip-end surface of an optical integrated circuit (refer to Patent-related Document 1). For example, in the case of a spot size converter, by making it to have a construction for coupling it with an end-surface of an external optical circuit and gradually reducing the width of the optical waveguide, it becomes possible to enhance integrity of an optical mode field with the external circuit. As a result, it becomes possible to optically couple signal light directly from an external optical circuit to input it into an optical waveguide.
FIG. 1 is a schematic diagram showing arrangement of a prior-art optical integrated circuit that uses a spot size converter such as that explained above (refer to Non-patent-related Document 2 also). As shown in the figure, in an optical integrated circuit 1, one end (end-surface coupling part 15) of a spot size converter 10 is connected to an external optical circuit 20 (here, it is supposed that this is a semiconductor laser), and another end is connected to an optical waveguide 30. Signal light entered, by optical coupling, from the external optical circuit 20 propagates through the optical waveguide 30 via the spot size converter 10, and the signal light is coupled to a multimode optical waveguide element 40 (here, it is supposed that this is a MMI coupler). Next, each part of the signal light, that is formed by dividing the signal light by the multimode optical waveguide element 40, propagates through a wide-width optical waveguide 50 (here, this is a multimode optical waveguide element that is made to have a wide waveguide width to reduce a propagation loss) via an optical waveguide 45, and the part of the signal light is inputted to an optical element such as an optical modulator 60. Further, the modulated light outputted therefrom is coupled to another optical element via an optical waveguide.
Note that an optical integrated circuit such as that shown in FIG. 1 can be formed to be in the shape of a square and to have a plane size such that each side of the square has a length of approximately 5 mm.